Snippet of FuturesB.java Example of using Futures for nested calls showing how it blocks inefficiently.

FuturesB-snippet.java

ExecutorService executor = new ThreadPoolExecutor(4, 4, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<Runnable>());

// get f3 with dependent result from f1
Future<String> f1 = executor.submit(new CallToRemoteServiceA());
Future<String> f3 = executor.submit(new CallToRemoteServiceC(f1.get()));

/* The work below can not proceed until f1.get()
   completes even though there is no dependency */

// also get f4/f5 after dependency f2 completes 
Future<Integer> f2 = executor.submit(new CallToRemoteServiceB());
Future<Integer> f4 = executor.submit(new CallToRemoteServiceD(f2.get()));
Future<Integer> f5 = executor.submit(new CallToRemoteServiceE(f2.get()));

System.out.println(f3.get() + " => " + (f4.get() * f5.get()));

The full example with various implementations is at https://gist.github.com/4671081